The creation and the distribution of cryptographic keys is a prerequisite for encrypted communications. Quantum key distribution (QKD) can be used to produce and distribute cryptographic keys, but not to transmit any message data. One significant advantage of QKD, compared to computationally secure key distribution techniques, is that there exist QKD protocols for which the security can be formally established in an information-theoretic setting. Even when some computational techniques are used for authentication purposes of the classical channel, QKD can guarantee everlasting security.
Everlasting security, for a key establishment protocol, in particular implies that once the protocol ends and the keys are distributed, their security cannot be jeopardized at any point in the future, irrespectively of the progresses in computing power or in cryptanalysis made by any potential attacker. Such strong security guarantee cannot be obtained with computational techniques.
Despite being capable of offering everlasting security, QKD presents limitations in terms of performance and resource requirements. As of today, QKD can be reliably deployed over metropolitan distances (below 80 kilometers). Long distance QKD is possible for distances up to a few hundreds of kilometers over optical fibers, but the achievable key rates are then low or insufficient for specific uses. It is moreover hard to deploy QKD over long distances with reliability as the requirements on detector noise, post-processing efficiency and system stability all increase with distance.
As a noticeable limitation, QKD is not compatible with optical amplifiers. As a consequence, in absence of reliable quantum repeaters, the distance reachable using QKD can only be extended beyond the reach of a single QKD link using classical trusted repeaters. Such repeaters require specific measures, in general complex and costly, to guarantee their security. QKD with trusted repeaters thus does not fit with the paradigm of end-to-end security and this constitutes a limitation. QKD presents other limitations.
The patent document WO2005046114, published in 2005 and entitled “Coherent-states based quantum data-encryption through optically-amplified WDM communication networks” discloses a quantum cryptographic protocol which uses two-mode coherent states that is optically amplifiable, resulting in a polarization independent system that is compatible with the existing WDM infrastructure and which provides secure data encryption suitable for wavelength division multiplexing networks through an in-line amplified line. The security of this scheme is intrinsically related to the security of an underlying cipher that is computationally secure. The security advantage of such scheme over computationally secure encryption, and in particular everlasting security cannot be established. Hence this approach presents limitations.
There is a need for methods and systems for communicating a message M between distant parties A and B with everlasting security, and in particular for which untrusted repeaters can be realized.